Manganiferous sea nodules found in large quantities on the ocean floor are recognized to be a potentially valuable source of metals. The nodules contain substantial amounts of manganese and iron and a minor amount of nonferrous metals such as nickel, cobalt and copper. Typical deposits may contain up to about 2% nickel, up to about 2% copper, up to about 1% cobalt, up to about 25% iron and up to about 40% manganese, by weight (on a dry basis). The physical and chemical nature of the deposits vary depending on their location. However, since these components are tied in intimate and complex association, they are not amenable to separation by conventional low-cost physical beneficiation procedures. For the same reason extraction of these valuable metals is difficult.
Several methods have been proposed for extracting the metal values. The merits and disadvantages of a particular process depend on many factors in addition to the nature of the nodules, which may vary considerably with location of the deposits. For example, the availability and cost of fuel are major considerations. Selectivity, extent of extraction of desired metals, speed of reaction, availability and cost of reagents, reagent consumption, and equipment requirements are among other factors to be considered. Thus, alternative routes for extracting metal values permit the choice of a method which will give optimum recovery of the desired metals at a minimum cost in the light of variable pertinent factors.
Among the processes proposed for extracting metal values from sea nodules are those involving direct leach of the nodules with sulfuric acid. Leaching with sulfuric acid at ambient temperatures and pressures is a slow process in which acid consumption is high and the final extraction of valuable metals may be incomplete. To improve the dissolution of the valuable nonferrous metals, reducing agents which will dissolve the tetravalent manganese have been utilized in conjunction with an acidic leach. For example, U.S. Pat. No. 3,169,856, uses sulfur dioxide to reduce the tetravalent manganese and simultaneously dissolve nickel and copper values. Subsequently, the residue is releached to recover cobalt and any remaining nickel and copper values. Another process, U.S. Pat. No. 3,795,596 leaches the nodules with sulfuric acid to dissolve a portion of the copper and nickel and then releaches the residue with a ferrous sulfate or ferrous chloride solution to dissolve the manganese and the remaining valuable nonferrous metals. A major disadvantage of such reductive leach processes is that the resultant leach solutions contain the bulk of the manganese along with at least a portion of the valuable nonferrous metals and possibly iron. Solution treatment schemes which require separating nickel, copper and cobalt from major amounts of manganese in solution before recovering or rejecting the manganese from solution are costly, and the overall cost of the process in comparison to the value of the products may be unattractive.
The use of a high temperature sulfuric acid leach for obtaining more rapid and complete extraction of the valuable nonferrous metals has been suggested. It has been found, for example, that at 450.degree. F. and 500 psig, nickel, copper, and cobalt can be rapidly extracted from sea nodules with lower acid consumption than required for the sulfuric acid leach at ambient conditions. Essentially, the same process has previously been applied to lateritic ores. In this process, nickel and cobalt are extracted from the lateritic ore by treatment with sulfuric acid in a steam agitated pachuca tank at 230.degree. to 260.degree. C. and about 400 to 600 psig steam pressure. Published information on the operation of this process indicates that formation of massive sulfate scale deposits in the leaching vessels is a major problem. We have found that leaching sea nodules under similar conditions also results in formation of scale deposits. This will necessitate frequent shut down of the leaching vessels to remove the deposits, thus adding to labor costs, and so on.
One way to solve the scale deposit problem which we have found is to increase the agitation of the slurry during leaching. The power input to prevent scale formation by mechanical agitation, however, is undesirably high.
Large industrial processes are, of course, preferably operated continuously rather than by batch leaching. In high temperature, high pressure processes, special expensive pumps are required to feed the ore slurry to the leaching vessels. Pumping acidic slurries through such pumps may cause excessive corrosion and is avoided. Therefore, sulfuric acid is normally added to the hot ore slurry in the autoclave through a separate stream.
It has now been found that by subjecting the sea nodules to a two-stage leach in which the sea nodules are subjected initially to acid leaching under relatively mild conditions and then to leaching at high temperature and pressure, the valuable nonferrous metals such as nickel, cobalt and copper can then be extracted effectively under conditions which permit relatively rapid extraction with minimized scale formation. Furthermore, since most of the sulfuric acid is consumed during the first stage leach, corrosion during pumping the resultant slurry (pH preferably .gtoreq.2) to the high temperature, high pressure autoclave is minimized. During the low temperature leach some of the nickel and copper values as well as soluble gangue components are dissolved. However, it is the second high temperature leach that is relied on for high extraction of the copper, nickel and cobalt.
It is an object of the present invention to provide a method for treating manganese sea nodules to selectively extract nickel, copper, and cobalt, thereby avoiding complex solution treatment procedures for separating such values from manganese. Another object is to provide a method in which neither thermal pretreatment nor drying of the ore is required. A further object is to provide a hydrometallurgical method for treating sea nodules in which a sulfuric acid leach is carried out under conditions which will avoid the formation of massive basic sulfate scale deposits in the leaching vessel, but will not require a long period of time for suitable extraction nor an excessive consumption of acid.
These and other objects will become apparent from the following description taken in conjunction with the accompanying drawing.